Ultrasensitive nanoporous interferometric sensor for label-free detection of gold(III) ions

Abstract

In this study, we present an ultrasensitive sensor based on nanoporous anodic alumina (NAA) for detection of gold(III) ions (Au³⁺ ions) using reflectometric interference spectroscopy (RIfS). Nanoporous anodic alumina, prepared by two-step electrochemical anodization, was functionalized with 3-mercaptopropyl-tirethoxysilane (MPTES) in order to selectively detect Au³⁺ ions. Thus prepared, MPTES-NAA sensors were exposed to different concentrations of Au³⁺ ions ranging from 0.1 to 750 μM and the changes in the effective optical thickness (ΔEOT) were monitored in real-time. The linear range of these Au³⁺ sensors was from 0.1 to 80 μM, with a lower detection limit of 0.1 μM of Au³⁺ ions. Furthermore, the specificity of these MPTES-NAA sensors was validated by sequential exposure to 40 μM solutions of Fe³⁺, Mg²⁺, Co²⁺, Cu²⁺, Ni²⁺, Ag⁺, and Pb²⁺, resulting in negligible changes in EOT as compared to the same concentration of Au³⁺ ions. Detection of Au³⁺ ions in complex and environmentally and biologically relevant solvents such as tap water and phosphate buffer solution (PBS) was also successfully carried out in order to demonstrate the real-life application of these sensors. Finally, the binding isotherm for Au³⁺ ions and thiol (SH) group of MPTES-NAA system was determined by fitting the changes in EOT to Freundlich and Langmuir isotherm models.